The low-frequency spectra of nonthermal radio sources

Roger, R. S.; Costain, C. H.; Bridle, A. H.

doi:10.1086/111506

Cited by 103 publications

(86 citation statements)

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“…We tested polynomial fits up to the fourth order and found that a first order polynomial function (A 0 = 1226 ± 17 and A 1 = −0.79 ± 0.008) is the best-fit model (as already pointed out by a number of authors, e.g. Roger et al 1973). We derived the expected total flux of Virgo A at the frequency of each observed LOFAR SB and rescaled the model used for that SB to match it at the beginning of each cycle of self-calibration.…”

Section: Absolute Flux Densitymentioning

confidence: 55%

“…To do that, we collected the total flux measurements available in the literature in the frequency range from 10 to 1400 MHz (Braude et al 1969;Bridle & Purton 1968;Roger et al 1969;Viner & Erickson 1975;Kellermann et al 1969;Wright & Otrupcek 1990). Each data-point was corrected to match the Roger et al (1973;RBC) flux scale with correction factors from Laing & Peacock (1980) and Scaife & Heald (2012). A model of the form log S = log(A 0 ) + A 1 log ν 150 MHz…”

Section: Absolute Flux Densitymentioning

confidence: 99%

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M 87 at metre wavelengths: the LOFAR picture

Gasperin

Orrù

Murgia

et al. 2012

A&A

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Context. M 87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4 × 10 9 M , whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Aims. To place constraints on past activity cycles of the active nucleus, images of M 87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M 87 extended radio-halo. Methods. We present the first observations made with the new Low-Frequency Array (LOFAR) of M 87 at frequencies down to 20 MHz. Three observations were conducted, at 15−30 MHz, 30−77 MHz and 116−162 MHz. We used these observations together with archival data to produce a low-frequency spectral index map and to perform a spectral analysis in the wide frequency range 30 MHz-10 GHz. Results. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intracluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radiospectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of 10 μG, which increases to 13 μG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of 40 Myr, which in turn provides a jet kinetic power of 6−10 × 10 44 erg s −1 .

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Section: Absolute Flux Densitymentioning

confidence: 55%

Section: Absolute Flux Densitymentioning

confidence: 99%

M 87 at metre wavelengths: the LOFAR picture

Gasperin

Orrù

Murgia

et al. 2012

A&A

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“…We see that there is a slight but clearly significant difference in both the peak and integrated flux densities in the 7C and MSSS catalogs, in the sense that the MSSS flux densities are systematically high by about 9% (total flux density) or 6% (peak flux density). Both surveys should be tied to the flux scale of Roger et al (1973), so in principle we would not expect this systematic difference. In practice, the difference is likely due to different assumptions about the flux density of 3C 295, the reference flux calibrator for almost all the MSSS observations, which seems likely to have been the primary flux calibrator for the 7C observations as well given the RA of the field (McGilchrist et al 1990).…”

Section: Hbamentioning

confidence: 99%

The LOFAR Multifrequency Snapshot Sky Survey (MSSS)

Heald

Pizzo

Orrù

et al. 2015

A&A

107

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We present the Multifrequency Snapshot Sky Survey (MSSS), the first northern-sky Low Frequency Array (LOFAR) imaging survey. In this introductory paper, we first describe in detail the motivation and design of the survey. Compared to previous radio surveys, MSSS is exceptional due to its intrinsic multifrequency nature providing information about the spectral properties of the detected sources over more than two octaves (from 30 to 160 MHz). The broadband frequency coverage, together with the fast survey speed generated by LOFAR's multibeaming capabilities, make MSSS the first survey of the sort anticipated to be carried out with the forthcoming Square Kilometre Array (SKA). Two of the sixteen frequency bands included in the survey were chosen to exactly overlap the frequency coverage of large-area Very Large Array (VLA) and Giant Metrewave Radio Telescope (GMRT) surveys at 74 MHz and 151 MHz respectively. The survey performance is illustrated within the MSSS Verification Field (MVF), a region of 100 square degrees centered at (α, δ) J2000 = (15 h , 69 • ). The MSSS results from the MVF are compared with previous radio survey catalogs. We assess the flux and astrometric uncertainties in the catalog, as well as the completeness and reliability considering our source finding strategy. We determine the 90% completeness levels within the MVF to be 100 mJy at 135 MHz with 108 resolution, and 550 mJy at 50 MHz with 166 resolution. Images and catalogs for the full survey, expected to contain 150 000-200 000 sources, will be released to a public web server. We outline the plans for the ongoing production of the final survey products, and the ultimate public release of images and source catalogs.

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“…We used the peak flux density of the sources in this catalogue instead of the integrated flux density because with a resolution of 54 any extended emission in WENSS will not contribute to the compact flux at ∼1 scales relevant to the LOFAR long-baseline calibration. We note that all WENSS peak flux densities in this paper include a correction factor of 0.9 with respect to the original catalogue to place them in the RCB scale (Roger et al 1973), as recommended by Scaife & Heald (2012). For the first observation we selected an area of 11.6 • radius centred on (18 h 30 m , +65 • ), or Galactic coordinates (l, b) = (94.84 • , +26.6 • ).…”

Section: Target Selectionmentioning

confidence: 99%

“…2 we show the distribution of flux density of the counterparts of the observed sources found in the VLSSr catalogue at 74 MHz, 4 m wavelength (Lane et al 2012(Lane et al , 2014 and the WENSS catalogue at 325 MHz, 92 cm wavelength. Like the (corrected) WENSS flux densities, the VLSSr catalogue flux scale is also set using the RCB flux density scale (Roger et al 1973). Based on these two values, we also show the distribution of the estimated flux density of the sources at 140 MHz.…”

Section: Target Selectionmentioning

confidence: 99%

The LOFAR long baseline snapshot calibrator survey

Moldón

Deller

Wucknitz

et al. 2015

A&A

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Aims. An efficient means of locating calibrator sources for international LOw Frequency ARray (LOFAR) is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz. Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full international LOFAR array. Sources were preselected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators. Results. More than 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the sky density of calibrators that are sufficiently bright to calibrate dispersive and non-dispersive delays for the international LOFAR using existing methods is 1.0 per square degree. Conclusions. The observed density of satisfactory delay calibrator sources means that observations with international LOFAR should be possible at virtually any point in the sky provided that a fast and efficient search, using the methodology described here, is conducted prior to the observation to identify the best calibrator.

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The low-frequency spectra of nonthermal radio sources

Cited by 103 publications

References 0 publications

M 87 at metre wavelengths: the LOFAR picture

M 87 at metre wavelengths: the LOFAR picture

The LOFAR Multifrequency Snapshot Sky Survey (MSSS)

The LOFAR long baseline snapshot calibrator survey

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